Alteration in the level of endogenous hypothalamic prostaglandins induced by delta 9-tetrahydrocannabinol in the rat

1 Whole brain and regional brain levels of prostaglandin E2 (PGE2)-like material have been determined following administration of delta 9-tetrahydrocannabinol (delta 9 -THC) in rats. 2 Intravenous administration of delta 9-THC 2 mg/kg, resulted in marked behavioural changes and hypothermia. The behavioural changes consisted mainly of catatonia (most apparent at 30 min after administration of delta 9-THC), followed by sedation (most evident at 60 min). Hypothermia was marked from 30 min after administration of delta 9-THC. 3 delta 9-THC did not after the whole brain levels of PGE2-like material 30, 60 or 120 min after administration. 4 delta 9-THC did not alter the levels of PGE2-like material in the medulla oblongata/pons, midbrain, cortex and cerebellum, 30 min after administration. However, there was a significant reduction of PGE2-like material in the hypothalamus, 30 min after delta 9-THC. 5 It is suggested that the delta 9-THC-induced decrease in hypothalamic PGE2-like material may contribute to the hypothermia observed following delta 9-THC administration.

Time-course of the effects of chronic delta 9-tetrahydrocannabinol on behaviour, body temperature, brain amines and withdrawal-like behaviour in the rat

The effects of clomipramine HCl (15 mg kg-1 i.p.) on behaviour, body temperature and brain amines were investigated in rats that had been chronically treated twice daily with increasing doses of delta 9-tetrahydrocannabinol (delta 9-THC, 2-6 mg kg-1 i.v.). delta 9-THC produced a biphasic change in behaviour, stimulation followed by depression, and a pronounced hypothermia. Tolerance developed rapidly to these effects of delta 9-THC. Chronic treatment with delta 9-THC reduced the levels of homovanillic acid, 5-hydroxytryptamine and noradrenaline. The level of dopamine was not altered with chronic treatment and tolerance appeared to develop to the increased levels of 5-hydroxyindoleacetic acid induced by delta 9-THC. Injection of clomipramine, 12-14 h after 2, 5 or 10 days of delta 9-THC treatment induced characteristic changes in the rats behaviour which consisted of writhes, backward kicking, wet shakes, jumps ataxia and front paw and whole body tremor. The severity of the behavioural changes appeared to be dependent on the period of delta 9-THC administration and they were not accompanied by a change in body temperature or consistent changes in brain amines or metabolites. The results indicate that physical dependence on delta 9-THC may occur since clomipramine is able to precipitate changes in behaviour, indicative on an abstinence syndrome, in rats chronically treated with delta 9-THC. It is suggested that tryptaminergic mechanisms are altered during chronic delta 9-THC treatment and that clomipramine induces the behavioural changes by interacting with an altered tryptaminergic system.

Neuropharmacological characterization of a taurine antagonist

6-Aminomethyl-3-methyl-4H-1,2,4-benzothiadiazine-1,1-dioxide hydrochloride (TAG) applied by microiontophoresis caused a prompt and readily reversible antagonism of the inhibitory effects of taurine and beta-alanine on rat somatosensory cerebral cortical neurons and cerebellar Purkinje neurons, although not affecting the actions of gamma-aminobutyric acid. TAG also consistently reduced, but did not abolish, the synaptically evoked inhibitions of Purkinje cells produced by electrical stimulation of the cerebellar surface. On the isolated amphibian spinal cord, TAG caused a dose-related enhancement of the dorsal root-ventral root potential, although not substantially affecting the dorsal root-dorsal root potential. Qualitative studies on the amphibian spinal cord showed that the depolarizing actions of taurine and beta-alanine on the dorsal roots, but not those of gamma-aminobutyric acid or glycine, were reduced by TAG. Somewhat surprisingly, TAG also offset the inhibitory effects on cortical cells and the dorsal root depolarizing actions of muscimol, 3-aminopropane sulfonic acid and piperidine-4-sulfonic acid, all of which heretofore have been considered as selective gamma-aminobutyric acid agonists. These findings would appear to lend a substantial degree of credence to the repeated suggestions that taurine may subserve a neurotransmitter-type function in the mammalian central nervous system. Furthermore, the availability of a relatively specific antagonist might reasonable be expected to facilitate efforts to elucidate the physiological functions of taurine not only in the central nervous system but in other organ systems in the body as well.

Attenuation of delta 9-tetrahydrocannabinol-induced withdrawal-like behaviour by delta 9-tetrahydrocannabinol

Rats chronically treated with increasing daily doses of delta 9-tetrahydrocannabinol (delta 9-THC) for up to 5 or 10 days exhibit a withdrawal-like behavioural response when challenged with clomipramine, a potent inhibitor of serotonin (5-hydroxytryptamine) reuptake, at the time when the next injection of delta 9-THC was due. To determine whether this response is indeed a withdrawal syndrome, different groups of rats were injected IV twice daily for up to 5 days with either delta 9-THC, in doses increasing from 2-6 mg/kg, or polyvinyl-pyrrolidone (PVP), the vehicle in which delta 9-THC was suspended. On day 6, an acute dose of 6 mg/kg delta 9-THC was given 30 min before IP clomipramine (15 mg/kg). The total behavioural response, which included writhing, backward kicking, jumping, head shaking, and paw tremor, was attenuated in rats chronically treated with delta 9-THC, but not in rats which received an acute dose of PVP. These results lend further evidence to our hypothesis that chronically administered delta 9-THC produces a state of physical dependence in the rat.

Isolation and determination of tissue spermidine using Bio-Rex 70

Bio-Rex 70, a weak cation exchange resin, has been used to extract spermidine from rat tissues. This method compares favourably to other extraction procedures with respect to selectivity, reproducibility and time needed to perform the procedure and, because it combines the optimum of these procedures, it appears more adaptable for routine laboratory use. Following extraction, spermidine is quantified fluorometrically after condensation with o-phthaldialdehyde (OPT). The sensitivity of the present procedure allows for the chromatographic isolation of 20 ng spermidine to give fluorescence twice that of blank. The procedure is selective for spermidine because the fluorescence of other OPT-reactive substances has been eliminated by either the extraction or the assay procedures. The stability and reproducibility of the adsorption and elution characteristics of Bio-Rex 70 enables the determination of 30 tissue samples in a working day. This procedure has been used to determine the levels of spermidine in whole and regional brain areas. In addition, the concentration of spermidine in peripheral tissues was also determined. Of the brain regions, the medulla oblongata/pons had the highest level of spermidine (68.24 +/- 7.67 microgram/g), and in peripheral tissues the highest level of this polyamine was found in the heart (48.59 +/- 4.32 microgram/g).

Electrophysiological actions of benzodiazepines

Electrophysiological investigations have revealed that benzodiazepines, applied either locally or systemically, reduce central nervous system excitability. The studies summarized here indicate that this depression of excitability by benzodiazepines is a result of an increase in gamma-aminobutyric acid (GABA) mediated inhibition. This increase in inhibition may result from benzodiazepines increasing the activity of some GABAergic neurons and also from a modulatory action of benzodiazepines on GABA actions at some postsynaptic receptor sites. The modulatory action is observed with doses of benzodiazepines that do not cause any direct effects on neuronal excitability or membrane polarization. Specificity tests indicate that benzodiazepines do not enhance inhibition mediated by glycine or monoamines such as norepinephrine or serotonin. Results of experiments with a convulsant benzodiazepine compound, which causes a specific reduction in GABA-mediated inhibition, are also presented, The data are discussed in terms of a model in which the benzodiazepine receptor, the GABA receptor, and the chloride ionophore are functionally linked. Furthermore, it is proposed that some postsynaptic actions of GABA may be continually regulated by the occupancy of a benzodiazepine receptor, and that occupancy of the benzodiazepine receptor may be permissive for the GABA-elicited increase in chloride ion permeability.

A modified method for the isolation and determination of brain histamine using the Bio-Rex 70

Bio-Rex 70, a weak cation exchange resin, has been used to specifically isolate histamine from rat brain tissue. This method compares favourably to other extraction procedures with respect to selectivity, reproducibility and time taken to perform the procedure. However, because it combines the optimum of these properties, it appears more adaptable for routine laboratory use. Following isolation, histamine is quantified fluorometrically after condensation with o-phthaldialdehyde (OPT). The sensitivity of the procedure allows for the chromatographic isolation, using Bio-Rex 70, of 12.5 ng histamine to give fluorescence twice that of blank. In addition, the use of Bio-Rex 70 enables the selective separation of histamine from fluorescent contaminants such as spermidine. The stability and the reproducibility of the adsorption and elution characteristics of Bio-Rex 70 enables the determination of 30 brain samples in a working day. This method has been applied to determine whole brain and regional brain levels of histamine in control and L-histidine-treated rats. The whole brain level of histamine, which was 50 ng/g, was increased by L-histidine and the highest concentration of histamine was found in the hypothalamus. Since the reliability of existing histamine extraction procedures is questionable, under certain conditions, it is suggested that the use of Bio-Rex 70 is a valuable addition in evaluating the possible physiological role of brain histamine.

The action of adenosine on noradrenergic neuronal inhibition induced by stimulation of locus coeruleus

Stimulation of locus coeruleus (LC) has been used to induce a noradrenergic inhibition of neurones in the rat cerebral cortex. The local application of adenosine or adenosine monophosphate by microiontophoresis in the cortex was found to produce a reduction of the evoked inhibition if the purine application was begun 20 sec before LC stimulation, but an enhancement of the inhibition if applied up to 35 sec after the LC stimulation. GABA increased the duration of LC inhibition irrespective of time of application. Adenosine and GABA showed no mutual potentiation, but norepinephrine increased the size of responses to both iontophoretic adenosine and GABA. The adenosine-norepinephrine interaction was synergistic, irrespective of the order of application. It is concluded that adenosine may act both presynaptically to inhibit, and postsynaptically to enhance the effects of noradrenergic neurone activation, the dominant effect observed depending on the temporal relationship between LC activation and adenosine application.

Withdrawal-like behaviour induced by inhibitors of biogenic amine reuptake in rats treated chronically with delta 9-tetrahydrocannabinol

The effects of the biogenic amine reuptake inhibitors fluoxetine, clomipramine and imipramine on the behaviour of rats after chronic treatment with delta 9-tetrahydrocannabinol (delta 9-THC) for 5 and 10 days were examined. Rats with permanently in-dwelling IV cannulae were injected twice daily with doses of delta 9-THC (2-6 mg/kg). delta 9-THC treatment reduced the rate of body weight gain and induced the typical biphasic modifications of behaviour. Tolerance developed to both of these effects. On days 6 and 11 of the experiment, rats were injected IP with 15 mg/kg imipramine HCl, clomipramine HCl or fluoxetine HCl, and behaviour, consisting of writhes, backward kicks, jumps and wet shakes, was observed for the next 30 min. Each of the amine reuptake inhibitors induced changes in behaviour, the severity of which appeared to correlate with their ability to inhibit the reuptake of 5-hydroxytryptamine (5-HT). It is suggested that tryptaminergic mechanisms are involved in the production of a withdrawal-like behaviour after chronic delta 9-THC treatment.

Seizures and related epileptiform activity in hippocampus transplanted to the anterior chamber of the eye: modulation by cholinergic and adrenergic input

Transplants of rat hippocampus into the anterior chamber of the eye of a host animal were used to assess the effects of cholinergic and adrenergic neuronal inputs on the generation and duration of seizure activity. Cholinomimetics initiated both seizures and hypersynchronous neuronal activity in the transplants. Cyclic guanosine monophosphate (GMP) derivatives and isobutyl methylxanthine elicited similar changes. Reflex activation of the cholinergic parasympathetic input to the iris and transplant by illumination of the ipsilateral retina also induced seizures or increased the rate of penicillin-induced interictal spike discharge. Application of beta-adrenergic agonists inhibited interictal spikes and paroxysmal depolarizing shifts induced by penicillin. Fluorescence histochemical studies showed that host sympathetic adrenergic fibers derived from the ground plexus of the iris invaded the transplant to form fine varicose nerve terminals. Activation of these adrenergic afferents to the transplant diminished both the amplitude and frequency of penicillin-induced epileptiform activity. Epileptiform activity in hippocampal occular transplants is strongly modulated by cholinergic and adrenergic neuronal inputs, with the former exerting a facilatory influence and the latter, an inhibitory effect.

The effect of (-)-trans-delta 9-tetrahydrocannabinol on regional brain levels and subcellular distribution of monoamines in the rat

1. The effect of intravenously injected delta 9-tetrahydrocannabinol (delta 9-THC, 2 mg/kg) on subcellular distribution in the whole brain and the regional brain levels of noradrenaline, dopamine, serotonin and 5-hydroxyindoleacetic acid were determined in the rat. 2. The levels of noradrenaline and dopamine were not altered by delta 9-THC in the hypothalamic, medullary and rest of brain areas, whereas those of serotonin and 5-hydroxyindoleacetic acid were elevated in the medullary and hypothalalmic areas, respectively. 3. delta 9-THC did not alter the levels of these monoamines and the metabolite 10 min after injection; however, there was a shift of dopamine from the bound to the free fraction. On the other hand, there was a shift of 5-hydroxyindoleacetic acid from the free to the bound fraction. 4. After 1 h, there was no difference in the subcellular ratios of noradrenaline, dopamine, serotonin and 5-hydroxyindoleacetic acid were increased. 5. It is suggested that the effects of delta 9-THC may be mediated by modification of the subcellular distribution of dopamine and serotonin.

The effect of delta 9-tetrahydrocannabinol (delta 9-THC) on the turnover rate of brain serotonin of the rat

1. One isotopic and three non-isotopic methods were used to determine the effect of an acute intravenous dose of delta 9-tetrahydrocannabinol (delta 9-THC, 2 mg/kg) on the rat brain turnover rate of serotonin. 2. In control animals the turnover rate of serotonin was about 2 nmol/g per h. This rate was not altered by delta 9-THC when it was calculated from the rise of 5-hydroxyindoleacetic acid following probenecid from the rise of serotonin following pargyline. 3. delta 9-THC did not alter serotonin turnover rate when it was calculated from the conversion of 3H-tryptophan to 3H-serotonin. 4. The serotonin turnover rate was significantly increased by delta 9-THC when the rate was calculated from the decline of 5-hydroxyindoleacetic acid following pargyline. 5. These results suggest that delta 9-THC does not alter the turnover of rat brain serotonin. The previously reported delta 9-THC-induced changes in body temperature and increased brain levels of 5-hydroxyindoleacetic acid may be mediated by some other mechanism such as interference by delta 9-THC of the vesicular binding of serotonin.

Antidepressant drugs potentiate suppression by adenosine of neuronal firing in rat cerebral cortex

Adenosine and AMP (5'-adenosine monophosphate) applied by microiontophoresis produced depression of neuronal firing rates in cerebral cortex. A number of antidepressant drugs including examples which are known not to affect noradrenaline uptake systems, potentiated the depressant purine effects. Noradrenaline responses were unaffected or reduced. Purines may therefore be important in the mechanism of action of antidepressant drugs.

Antagonism by clonidine of neuronal depressant responses to adenosine, adenosine-5'-monophosphate and adenosine triphosphate

1. Adenosine and its nucleotides adenosine-5'-monophosphate (AMP) and adenosine triphosphate (ATP) have been applied by microiontophoresis to neurones in the cerebral cortex of rats anesthetized with urethane. The firing rate of most neurones was depressed, though two cells were encountered which showed biphasic responses to ATP consisting of an initial excitation succeeded by depression. 2. The application of clonidine with iontophoretic currents of less than 25 nA resulted in blockade of the depressant responses to the purines, without affecting responses to noradrenaline, 5-hydroxytryptamine or gamma-aminobutyric acid (GABA). At much higher doses of clonidine, direct depression of cell firing occurred and occasional interaction with noradrenaline was noted. 3. In the case of the biphasic responses to ATP, clonidine seemed to block only the depressant phase. Reduction of the excitatory component paralleled changes of background firing. 4. It is concluded that, in common with some other 2-substituted imidazoline derivatives, clonidine possesses the ability to block responses to purine compounds.

Benzodiazepines and central inhibitory mechanisms

The effect of diazepam was evaluated on spontaneous activity and drug- and electrically-elicited inhibitions of neuronal activity. Doses of diazepam which did not change spontaneous firing rates markedly enhanced GABA-mediated inhibitions in rat cerebellum in situ and in tissue cultures of rat hypothalamus. The effects of diazepam were readily reversible, and could be antagonized by picrotoxin; no effect on glycine or norepinephrine-induced inhibition was seen. It is concluded that actions of diazepam are mediated, at least in part, by a specific increase in GABA-mediated inhibition in the central nervous system.